唔.....呢度有個解答:
https://www.home-barista.com/tips/how-to-get-better-crema-with-non-pressurized-portafilter-t1917.html
"That teenie tiny hole
introduces air, producing a "fake" crema. The marketing people probably figure it's better to have something visible they can call crema than nothing, since it takes fresh coffee, a good grinder, and good technique to produce "real" crema. Like learning to cook a fine meal, many consumers don't have the patience, funds, or interest in such ventures, they just want a fast cuppa joe..."
"The pressurized portafilter essentially injects air into the mix while a non-pressurized basket relies on the proper balance of resistance from the coffee and it containing enough oils."